6fb614920b
As Peter pointed out, task_work() can avoid ->pi_lock and cmpxchg() if task->task_works == NULL && !PF_EXITING. And in fact the only reason why task_work_run() needs ->pi_lock is the possible race with task_work_cancel(), we can optimize this code and make the locking more clear. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
128 lines
3.4 KiB
C
128 lines
3.4 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/spinlock.h>
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#include <linux/task_work.h>
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#include <linux/tracehook.h>
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static struct callback_head work_exited; /* all we need is ->next == NULL */
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/**
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* task_work_add - ask the @task to execute @work->func()
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* @task: the task which should run the callback
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* @work: the callback to run
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* @notify: send the notification if true
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*
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* Queue @work for task_work_run() below and notify the @task if @notify.
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* Fails if the @task is exiting/exited and thus it can't process this @work.
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* Otherwise @work->func() will be called when the @task returns from kernel
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* mode or exits.
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*
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* This is like the signal handler which runs in kernel mode, but it doesn't
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* try to wake up the @task.
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*
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* Note: there is no ordering guarantee on works queued here.
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*
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* RETURNS:
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* 0 if succeeds or -ESRCH.
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*/
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int
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task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
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{
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struct callback_head *head;
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do {
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head = READ_ONCE(task->task_works);
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if (unlikely(head == &work_exited))
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return -ESRCH;
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work->next = head;
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} while (cmpxchg(&task->task_works, head, work) != head);
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if (notify)
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set_notify_resume(task);
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return 0;
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}
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/**
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* task_work_cancel - cancel a pending work added by task_work_add()
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* @task: the task which should execute the work
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* @func: identifies the work to remove
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*
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* Find the last queued pending work with ->func == @func and remove
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* it from queue.
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*
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* RETURNS:
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* The found work or NULL if not found.
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*/
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struct callback_head *
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task_work_cancel(struct task_struct *task, task_work_func_t func)
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{
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struct callback_head **pprev = &task->task_works;
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struct callback_head *work;
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unsigned long flags;
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if (likely(!task->task_works))
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return NULL;
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/*
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* If cmpxchg() fails we continue without updating pprev.
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* Either we raced with task_work_add() which added the
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* new entry before this work, we will find it again. Or
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* we raced with task_work_run(), *pprev == NULL/exited.
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*/
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raw_spin_lock_irqsave(&task->pi_lock, flags);
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while ((work = READ_ONCE(*pprev))) {
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if (work->func != func)
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pprev = &work->next;
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else if (cmpxchg(pprev, work, work->next) == work)
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break;
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}
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raw_spin_unlock_irqrestore(&task->pi_lock, flags);
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return work;
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}
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/**
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* task_work_run - execute the works added by task_work_add()
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*
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* Flush the pending works. Should be used by the core kernel code.
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* Called before the task returns to the user-mode or stops, or when
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* it exits. In the latter case task_work_add() can no longer add the
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* new work after task_work_run() returns.
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*/
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void task_work_run(void)
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{
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struct task_struct *task = current;
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struct callback_head *work, *head, *next;
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for (;;) {
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/*
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* work->func() can do task_work_add(), do not set
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* work_exited unless the list is empty.
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*/
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do {
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head = NULL;
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work = READ_ONCE(task->task_works);
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if (!work) {
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if (task->flags & PF_EXITING)
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head = &work_exited;
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else
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break;
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}
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} while (cmpxchg(&task->task_works, work, head) != work);
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if (!work)
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break;
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/*
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* Synchronize with task_work_cancel(). It can not remove
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* the first entry == work, cmpxchg(task_works) must fail.
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* But it can remove another entry from the ->next list.
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*/
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raw_spin_lock_irq(&task->pi_lock);
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raw_spin_unlock_irq(&task->pi_lock);
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do {
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next = work->next;
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work->func(work);
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work = next;
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cond_resched();
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} while (work);
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}
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}
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